Schema for configurations and feature flags.

A portable YAML/JSON format, an evaluation engine, and a visual configurator. No platform required.

Get started Try the configurator

npm install showwhat

showwhat.yeojz.dev/configurator

checkout_v2boolean

trueenv = prod

falsedefault

Preview

env: "prod"true

env: "staging"false

MIT Licensed

Minimal Dependencies

Multi-Runtime

TypeScript First

1 library, 1 file, 0 platform to operate.

Like OpenAPI and Swagger, but for feature flags.
A specification-first approach where the definition file is the source of truth, not a vendor database.

Portable format

Definitions are plain YAML or JSON. Store them in Git, serve from an API, or embed inline. The format stays the same everywhere.

Extensible conditions

Built-in primitives for strings, numbers, datetimes, and booleans.
Shorthand types for environments and time windows. Add your own with registerEvaluators().

Git-native workflow

PR reviews are your approval process. Commit history is your audit log. Branch protection is your RBAC. No separate dashboard to operate.

Define once, evaluate anywhere.

The definition format is the contract between your config authors and your runtime. Write it in YAML or JSON, evaluate it with one function call.

flags.yaml

definitions:
  checkout_v2:
    variations:
      - value: true
        conditions:
          - type: env
            value: prod
      - value: false

{
  "definitions": {
    "checkout_v2": {
      "variations": [
        {
          "value": true,
          "conditions": [
            { "type": "env", "value": "prod" }
          ]
        },
        { "value": false }
      ]
    }
  }
}

app.ts

import { showwhat, MemoryData } from "showwhat";

const data = await MemoryData.fromYaml(yaml);
const result = await showwhat({
  key: "checkout_v2",
  context: { env: "prod" },
  options: { data },
});

result.value; // true

import { showwhat, MemoryData } from "showwhat";

const data = MemoryData.fromObject(JSON.parse(json));
const result = await showwhat({
  key: "checkout_v2",
  context: { env: "prod" },
  options: { data },
});

result.value; // true

Load definitions from memory, files, or your own API. The DefinitionReader interface stays the same.

Time windows are config, not cron jobs.

Rules resolve top-to-bottom. Time windows, environment matching, and catch-all defaults compose in the same definition. No special syntax. No separate scheduling interface.

01:00 UTCnull

02:00 UTC"Deployment in progress"

04:30 UTC"Deployment in progress"

07:00 UTCnull

12:00 UTCnull

flags.yaml

definitions:
  maintenance_mode:
    variations:
      - value: "Deployment in progress"
        conditions:
          - type: env
            value: prod
          - type: startAt
            value: "2026-03-03T02:00Z"
          - type: endAt
            value: "2026-03-03T07:00Z"
      - value: null

{
  "definitions": {
    "maintenance_mode": {
      "variations": [
        {
          "value": "Deployment in progress",
          "conditions": [
            { "type": "env", "value": "prod" },
            { "type": "startAt", "value": "2026-03-03T02:00Z" },
            { "type": "endAt", "value": "2026-03-03T07:00Z" }
          ]
        },
        { "value": null }
      ]
    }
  }
}

Works everywhere JavaScript runs.

The core uses Web Standard APIs only. Zero Node.js-specific dependencies. Pick your runtime, import, and go.

import { showwhat, MemoryData } from "showwhat";
import { readFile } from "node:fs/promises";

const yaml = await readFile("./flags.yaml", "utf-8");
const data = await MemoryData.fromYaml(yaml);
const result = await showwhat({
  key: "checkout_v2",
  context: { env: "prod" },
  options: { data },
});

import { OpenFeature } from "@openfeature/server-sdk";
import { ShowwhatProvider } from "@showwhat/openfeature";
import { MemoryData } from "showwhat";

const data = await MemoryData.fromYaml(yaml);
await OpenFeature.setProviderAndWait(
  new ShowwhatProvider({ data })
);

const client = OpenFeature.getClient();
const result = await client.getBooleanValue(
  "checkout_v2", false, { env: "prod" }
);

import { showwhat, MemoryData } from "showwhat";

const yaml = await fetch(CONFIG_URL).then(r => r.text());
const data = await MemoryData.fromYaml(yaml);

const result = await showwhat({
  key: "checkout_v2",
  context: { env: "prod" },
  options: { data },
});

import { showwhat, MemoryData } from "npm:showwhat";

const yaml = await Deno.readTextFile("./flags.yaml");
const data = await MemoryData.fromYaml(yaml);

const result = await showwhat({
  key: "checkout_v2",
  context: { env: "prod" },
  options: { data },
});

import { showwhat, MemoryData } from "showwhat";

const yaml = await Bun.file("./flags.yaml").text();
const data = await MemoryData.fromYaml(yaml);

const result = await showwhat({
  key: "checkout_v2",
  context: { env: "prod" },
  options: { data },
});

Write your first definition now.

npm install showwhat

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